The present invention relates to the transmission of high frequency signals on optical fibers, and more particularly to conversion of radio frequency (RF) signals to light wave frequency signals, and back again, so that the signals can be carried by an optical fiber for subsequent reception at an optical detector.
Radio frequency signals may be collected at an antenna site that may be remote from a receiver, and the received signal must be conveyed from the antenna to the receiver. Typically, a received microwave signal is conveyed to the receiver in a coaxial cable or waveguide. For example, a ship may have an antenna platform where signals are collected on multiple antennas and routed in separate waveguides to receivers several decks below. However, metal coaxial cables and waveguides may be heavy, bulky and may distort the signals because they cannot carry the requisite bandwidth, and a substitute for the waveguides and cables has long been sought.
One suggested solution is to use optical fiber to carry the received signals to the receivers. Optical fiber is light weight, not bulky, and has more than adequate bandwidth. By way of example, and with reference to FIG. 1, a prior art system 10 may include a modulator 12 for modulating a received RF signal on an optical frequency carrier, such as from a laser 14. The modulated carrier signal may be conveyed via optical fiber 16 to a photodetector/demodulator 18 that demodulates the signal, generating the original RF signal for reception by a receiver 20.
Attempts to boost the modulated optical frequency carrier signal to provide sufficient gain to overcome transmission losses have not been successful because the carrier component of the signals is generally so large relative to the modulated signal that amplification sufficient for the modulated signal will cause the carrier to saturate an amplifier.
Further, as is apparent from FIG. 1, the photodetector/demodulator 18 must have a bandwidth large enough for all received signals (e.g., an entire transmission band). The compromises in performance attendant with this large bandwidth requirement make a narrower bandwidth desirable, although a suitable solution has not been found heretofore.
Accordingly, it is an object of the present invention to provide a novel system and method for converting RF signals to optical frequency signals that obviates the problems of the prior art.
It is another object of the present invention to provide a novel system and method for conveying a signal by optical fiber that has been received on a radio frequency in which the signal is modulated to optical frequency and subsequently transmitted with the carrier suppressed so that the optical frequency signal can be amplified before further transmission through an optical fiber.
It is yet another object of the present invention to provide a novel system and method for conveying a signal by optical fiber that has been received on a radio frequency in which the signal is modulated on the sideband of an optical frequency carrier in which the carrier has been suppressed.
It is a further object of the present invention to provide a novel system and method for conveying a radio frequency signal by optical fiber in which a Mach-Zehnder interferometer modulator suppresses the optical frequency carrier and modulates the signal on the double sidebands of the optical frequency carrier.
It is yet a further object of the present invention to provide a novel system and method for conveying a signal by optical fiber in which the optical signal is converted to an intermediate frequency before being demodulated.
It is still a further object of the present invention to provide a novel system and method for conveying a signal by optical fiber in which the optical signal is converted to an intermediate radio frequency by mixing with a tunable laser local oscillator.
It is an additional object of the present invention to provide a novel system and method for conveying a signal by optical fiber in which an optical bandpass filter suppresses an image of the intermediate radio frequency in the double sidebands conveyed by the optical fiber.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.